Various types of touch panels based on different working principles are known, and there are those using a large number of electrodes arranged in a panel for detecting a touch by using a resistive membrane or electrostatic capacitor sheet. In such cases, as the electrodes may function as aerial antennas, interferences from external electromagnetic noises may become a problem. In particular, in the case of capacitive touch panels, as the touch location is detected by a minute change in the capacitance near the corresponding electrodes caused by the approach or contact of a conductive object (such as a human body), noises may prevent accurate identification of the touch location, and an effective countermeasure against the problem of noises is desired.
JP 63-174120 (patent document 1) and JP 2010-009439 (patent document 2) disclose a technology for removing noises caused by the display device that is used in conjunction with the capacitive type touch panel device. It was also proposed in JP 2009-237637 (patent document 3) to use a shield wire to protect a capacitive type touch panel device from the influences of external noises. JP 02-176922 (patent document 4) discloses a technology for removing induction noises caused by the induction voltage induced by the user's finger.
The touch panel device is widely used in conjunction with personal computers and personal digital terminals, but may also be used in interactive white boards that are used for presentations and lectures for a large number of audiences in combination with a large screen display device. In such a case, if the touch panel device is of the capacitive type, in particular mutual capacitive type, a plurality of touch positions or multiple touch positions can be detected at the same time, and this enhances the convenience of the touch panel device.
However, as the mutual capacitive type touch panel device is susceptible to external noises as it is designed to detect a touch position by detecting an extremely small current generated in the reception electrode, and therefore involves a high impedance. In particular, as the length of the electrodes is increased owing to the increase in the size of the touch panel device, the problem of external noises become even more acute.
Also, the transmission electrodes are grouped into a number of groups, and each group is connected to an individual transmission circuit board via lead wires. The length of each lead wire increases as the size of the touch panel device increases. If the number of the transmission electrodes in each group is increased with the aim of reducing the number of the transmission circuit boards, a large part of the lead wires inevitably extend in parallel with the reception electrodes. As a result, the electric current of the drive signal that is conducted by the lead wires to be applied to the transmission electrodes may induce an undesirably high induction noises in the reception electrodes. The induction noises that are generated in the reception electrodes caused by the electric current conducted by the lead wires cannot be effectively removed by the conventional arrangement.
Also, the length of each lead wire varies from one another depending on the positions of the transmission electrodes and transmission circuit boards. The greater the distance between the transmission electrode and transmission circuit board is, the greater are the induction noises that are caused in the corresponding lead wire.